This invention relates generally to an improved viscosimeter of the type used in the drilling industry. In one aspect it relates to the method and apparatus for measuring the funnel viscosimeter of a drilling fluid. Another aspect of the invention relates to an improved Marsh funnel.
Consistent with the terminology used in the drilling industry, the terms used herein will have the following meanings:
“viscosity” is broadly defined as a resistance to flow;
“funnel viscosity” is defined as the time (seconds) required for a predetermined volume (one quart or one liter) of mud to flow out of a funnel viscosimeter into a graduated mud cup;
“mud” is defined as a water-based or oil-based drilling fluid containing gelling agents and other additives used in the drilling of oil wells, water wells, gas wells, injection wells, horizontal drilling, and all other areas of the industry.
Viscosity is one of the most important properties of a mud used in the rotary drilling methods. For this reason the mud is tested periodically (usually several times a day) to insure that the viscosity meets the design specification for the drilling fluid. In field operations, a funnel viscosimeter (known as the Marsh funnel) is the industry standard (see API Specification 13-B) and is used extensively throughout the drilling industry. Its use is simple, quick, fool-proof, and thoroughly familiar to field personnel.
The Marsh funnel is a conical-shaped funnel of specific dimensions (defined below) and has become the standard to the extent that the term “Marsh viscosity” or “Marsh funnel viscosity” is synonymous with funnel viscosity.
The funnel viscosity provides an indication of the viscosity (sometimes referred to as “apparent viscosity”) but does not provide a measurement of viscosity in conventional units. An article entitled “The Marsh Funnel and Drilling Fluid Viscosity: A New Equation for Field Use” [(SPE Drill & Completion 15, Vol. 15, No. 1 (March 2000)] correlated Marsh funnel measurements with conventional viscosity units. The disclosure of this article is incorporated herein by reference including the references cited therein. Although the Marsh funnel results are purely empirical, it is still widely used throughout the world.
The use of the Marsh funnel, however, presents certain operational problems. The operator responsible for testing the mud generally uses the following procedure: the mud sample is collected by placing the funnel in the drilling mud stream, (usually in the sample box), the mud being retained in the funnel by the operator holding his index finger over the funnel outlet orifice. In measuring the funnel viscosity, the operator releases or removes his finger from the opening while measuring the efflux time of the mud from the funnel. The number of seconds measured for the discharge of a quart (or liter) of mud from the funnel is the funnel viscosity.
This sampling and testing procedure is not as simple or fool-proof as it seems. To begin with, the operator is exposed to drilling mud. For deep wells, the mud is hot and can be painful to the touch. Also, the sampling and testing is cumbersome. Carrying the mud-filled funnel to the testing place requires both hands, one hand being used to carry the funnel and the other hand holding a finger over the orifice. At the testing place, the operator places the funnel in a stand with his finger retained over the orifice and then operates a stop watch with his free hand. Thus, it will be appreciated by those skilled in the drilling industry that this simple procedure not only ties up the operator for several minutes several times a day but presents risks of spillage or leakage from the funnel. This can lead to inaccuracies in the measurements. Also, the operator's movement around a slippery and muddy rig with both hands occupied is clearly an unsafe practice. In summary, there is a need to improve the funnel viscosimeter while retaining the simplicity and the reliability of the procedure.